Chemical etching, in a metallurgical sense, can be defined as a method for revealing structural details by the preferential attack of a metal surface with a chemical agent having a different effect on various alloy constituents. Many purposes for subjecting a metallic surface to an etching process are known and commonly practiced, including the inspection of metals for structural surface imperfections.
One test utilized for non-destructive detection of minute surface flaws and defects in metals is fluorescent penetrant inspection. This test includes applying to a metal surface a fluorescent liquid penetrant which enters into surface discontinuities or defects, removing any excess penetrant from the surface, and identifying the defect location by the emission of visible fluorescent light emitted by the retained penetrant upon exposure to ultraviolet light.
Prior to employing this process, it is necessary to chemically etch the metallic surface to remove any metal which, due to processes such as machining or grinding, has smeared over the surface and therefore masked possible flaws. These flaws, during subsequent processing or use of the metal, may propagate and cause premature failure if undetected. The etchant utilized in this treatment step must remove the surface metal within a prescribed time but must not attack grain boundaries or overetch the surface, thereby making the surface inadequate for accurate fluorescent penetrant inspection.
A commonly utilized pre-penetrant etchant includes a mixture of 50% hydrochloric acid and 50% hydrogen peroxide; however, this etchant has many serious drawbacks. Initially, use of the HC1/H.sub.2 O.sub.2 etchant often produces etch-induced artifacts, such as pitting of the metal hardware surface. Subsequent penetrant inspection and detection of these etch-induced flaws results in rejection of the metal hardware and subsequent reworking the hardware to remove the flaws.
A disadvantage related to the 50% HC1 - 50% H.sub.2 O.sub.2 etch is the volatile, unstable nature of the etchant composition itself. Because of this instability, the mixture must be formed immediately prior to use in a metal shop where shop personnel may not accurately prepare the mixture. The result is an imperfect concentration ratio which will greatly affect the etching process and cause the drawbacks caused above.
A clear need exists for an etchant composition which can be produced easily and quickly, which is storable, stable and has a long shelf life and which, when used prior to a penetrant inspection testing method, provides a surface acceptable for penetrant inspection without inducing additional flaws.